Extend NSString further and use the new methods

Extend NSString further and use the new methods

* src/nsfns.m (ns_set_icon_name):
(ns_set_name):
(ns_set_represented_filename):
(ns_implicitly_set_icon_type):
(ns_set_icon_type):
(ns_appkit_version_str):
(Fx_create_frame):
(Fns_read_file_name):
(Fns_get_resource):
(Fns_set_resource):
(Fns_list_colors):
(Fns_perform_service):
(ns_do_applescript): Use the new NSString methods.
([NSString stringWithLispString:]): Fix the surrogate algorithm.
([NSString lispString]): New method.
* src/nsterm.h (NSString): Add new method.
* src/nsterm.m ([EmacsApp openFile:]):
([EmacsApp requestService:userData:error:]):
([EmacsApp fulfillService:withArg:]):
([EmacsView changeFont:]):
([EmacsView setMarkedText:selectedRange:]):
([EmacsView initFrameFromEmacs:]):
([EmacsView performDragOperation:]):
([EmacsView performDragOperation:]):
([EmacsView performDragOperation:]):
([EmacsWindow accessibilityAttributeValue:]): Use the new NSString
methods.